CniT/roN. — Crustacea of the Kermailec Jalamh. 559 



Percnon pilimanus (A. Milne-Edwards). 



Acanthopus pilimanus A. M. -Edwards, Nouv. Archiv. Mus., 9, p. 300, 

 pi. 14, fig. 5, 1873. Lciolophus pilimanus Miers, Ann. Mag. Nat. 

 Hist., ser. 5, 1, p. IS-l, 1878. Percnon pilimanus Rathbun, V>\\\\. 

 U.S. Fish. Comm. for 1903, p. 842, 1906. 



Two males and several females from Sunday Island. 



These specimens agree very closely indeed with Milne-Edwards's descrip- 

 tion, except that there is no large tuft of fine hairs on the propod of the 

 chelipeds ; a well-marked tuft is, however, present on the merus of the 

 larger male. The width of the abdomen of the larger male at the base 

 is just equal to that of its length to the base of the last segment ; in the 

 smaller male the width is rather greater than this. 



It is possible that these specimens should be referred to P. planissimus, 

 but I assign them to P. pilimanus owing to the slightly narrower abdomen, 

 the spines on the inner margins of the antennulary cavities, and to the fact 

 that they agree minutely with Milne-Edwards's description and figure except 

 as regards the hairs on the propod of the chelipeds. The smaller speci- 

 mens agree well with the description of P. planissimus given by Alcock 

 (Journ. Asiatic Soc. Bengal, 69, p. 439), except that the second row of 

 spinules on the merus of the legs is well marked on the third legs as well 

 as on the first and second ; in the larger specimens the row is also indis- 

 tinctly marked on the fourth. Miss Rathbun records both species from 

 the Hawaiian Islands without comment. 



Milne-Edwards states that the hairs on the chelipeds act as a sponge 

 to maintain the humidity at the orifice of the branchial chamber. He had, 

 however, seen only one male specimen, and, as the tuft on the merus is 

 very small or quite absent in my female specimens, it seems more likely 

 that the hairs may be a sexual character, developed in the adult male 

 only, and in that case may not yet be fully developed in the two males in 

 my possession. The females resemble the male except in the much smaller 

 size of the chelipeds, which are much shorter and have the propod only 

 slightly widened. The merus bears only a very small tuft of hairs in the 

 larger female specimens, and none in the smaller specimens. It seems 

 likely, therefore, that the tufts of fine hairs on the merus and propod are 

 a secondary sexual character, developed only in large males, or perhaps 

 only during the breeding season ; they were evidently not present in the 

 adult males of P. planissimus examined by Alcock, for he makes no men- 

 tion of them. If, as seems likely, the other characters — i.e., the narrower 

 abdomen and the spines on the inner margin of the antennulary cavities — 

 do not prove to be constant, one would be tempted to suggest that P. planis- 

 simus and P. pilimanus form one species, in which the males may develop 

 the tufts of fine hairs on the chelipeds at certain seasons only. 



Male : Width of carapace, 29 mm. ; length, 32 mm. : total length of 

 propod of chehped, 15mm.; width, 11mm. Largest female: Width of 

 carapace, 28 mm. ; length, 31 mm. : total length of propod of cheliped, 

 8 mm. ; width, 6 mm. 



Mr. Oliver makes the following observations on the habits of this 

 species : " Fairly common among rocks near low-tide mark. Very quick 

 in its habits. Its colour somewhat resembles the rock, on which it stays 

 perfectly still, but when any one approaches too near it darts into the water. 

 When, after continued westerly winds, sand was driven ashore so as to 

 bury the boulders on the north coast of Sunday Island to about half-tide 



